Practitioners in the field of semiconductor device fabrication have long recognized the need to determine the sub-surface, compositional profiles of semiconductor wafers that have been processed, or partially processed. For example, dopant distributions produced by ion implantation followed by annealing are sensitive to process conditions such as implantation energy and the temperatures and durations which characterize the annealing cycle. It is important to measure the resulting distributions in order to establish appropriate process conditions. it is also important to measure the distributions from time to time during actual production of processed wafers, in order to assure that the manufacturing process is well controlled. For purposes of process initialization and control, it is important to measure the distributions in depth as well as in lateral dimensions.
As a further example, it is desirable during processing of, e.g., silicon wafers to diagnose the electronic properties at each processed layer within the wafer. For example, it may be desirable to distinguish layers at various depths which variously comprise crystalline, polycrystalline, and amorphous silicon.
Conventional techniques for the depth profiling of semiconductor substrates generally involve successive measurements, each performed after an incremental thickness has been removed from the wafer surface by ion sputtering or etching. The measurements performed by conventional, analytical instruments include, for example, capacitance measurements and direct analysis by secondary ion mass spectroscopy. Such techniques, however, are destructive and time-consuming. Especially for purposes of process monitoring and control, it is desirable to provide a depth profiling technique which non-destructive, and still more desirable to provide a technique which is also rapid. Such a technique is even more desirable if it can also be used for profiling in the lateral dimensions.
Non-destructive techniques have been reported for evaluating the compositions of semiconductor wafers at and very close to the wafer surface. Such techniques scan the surface of the sample with a modulated light beam (to be referred to as a "source beam"), and monitor the resulting time-dependent changes in reflectivity with a second light beam (to be referred to as a "probe beam"). The results of such measurments can be interpreted to yield information about compositional variations in depth as well as in the lateral dimensions. For example, U.S. Pat. No. 4,636,088, issued to A. Rosencwaig, et al. on Jan. 13, 1987, describes the use of the modulated beam to periodically heat the wafer surface. Because heating of the sample surface will change its reflectivity in a manner which is related to the local composition, compositional information can be deduced from the resulting modulations in the reflected probe beam. A second example is described in U.S. Pat. No. 5,042,952, issued to J. Opsal, et al. on Aug. 27, 1991. Described therein is a method in which the source beam is used to generate a periodic electron-hole plasma which interacts with features in the wafer as it diffuses. The resulting changes in the plasma density cause local alterations in the refractive index which, in turn, affect the reflectivity. The probe beam samples the plasma-induced, periodic reflectivity changes, yielding compositional information.
Although useful, the above-described optical methods are somewhat limited in spatial resolution in the depth dimension because they rely upon absorption of energy from the source beam. Therefore, those techniques are inherently limited to materials in which the source beam has a short penetration depth, which imposes a limit on the depth that can be probed. Moreover, the information which is collected by the probe beam represents an average in the depth dimension, down to the penetration depth. Therefore, it is not possible to achieve high spatial resolution in the depth dimension.